1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. The present invention more specifically relates to a reader intended for transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to a terminal only reading the data of a read-only transponder as well as to a read/write terminal adapted to modifying data contained in the transponder.
The present invention more specifically relates to the adaptation of the transmission power of a read/write terminal as a function of the distance from the transponder to the terminal.
2. Discussion of the Related Art
Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal.
FIG. 1 very schematically shows, in a simplified way, a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is formed of a series oscillating circuit, formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3 (generally, the ground). The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and exploiting received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. In the example shown in FIG. 1, node 5 of connection of capacitor C1 with inductance L1 forms a terminal for sampling a data signal received for the demodulator. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, includes an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), the outputs of which define D.C. supply terminals of the circuits internal to the transponder.
The oscillating circuit of terminal 1 is excited by a high-frequency signal (for example, at 13.56 MHz) which, in the absence of any data transmission from the terminal to the transponder, is exclusively used as a power source for the latter. When a transponder 10 is in the field of terminal 1, a high-frequency voltage is generated across terminals 11, 12 of the transponder's resonant circuit. This voltage, after being rectified and possibly clipped, is intended to provide the supply voltage for electronic circuits 13 of the transponder. These circuits generally include a microprocessor, a memory, a demodulator of the signals possibly received from terminal 1, and a modulator for transmitting information to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on the frequency of a transmission carrier, that is, the resonance frequency is set on a frequency of, for example, 13.56 MHz. This tuning aims at maximizing the energy diffusion to the transponder, generally, a card of credit card size integrating the different transponder components.
The high-frequency remote supply carrier transmitted by terminal 1 is also used as a data transmission carrier. This carrier is generally amplitude modulated by the terminal according to various coding techniques to transmit the data to the transponder. In return, the data transmission from the transponder to the terminal is generally performed by modulating the load formed by resonant circuit L2, C2. This load variation is performed at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal in the form of an amplitude variation or of a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of the current in the oscillating circuit. The data transmission, be it from the terminal to the transponder or from the transponder to the terminal, uses well known techniques that will not be detailed any further. It should only be noted that these data transmissions use, as a transmission carrier, the high-frequency transponder remote supply signal, even if the data transmitted by the terminal are modulated on a sub-carrier.
The voltage sensed by transponder 10 in the field of a terminal 1 depends on the distance separating the transponder from the terminal and, more specifically, on the coupling coefficient between the respective oscillating circuits of the terminal and of the transponder. To have a system with a relatively wide range (on the order of 4 to 8 inches), significant power has to be provided to the oscillating circuits of the terminal so that the radiated magnetic field remains sufficiently intense at the desired range distance to provide the necessary remote supply power to the transponder. However, this has the disadvantage that, when a transponder is close to the terminal, it receives too much power as compared to its needs. In addition to the fact that this requires providing means of protection against overvoltages on the transponder side, this causes a useless power overconsumption by the read/write terminal.
Another problem resulting from the high power radiated by the terminal is that several transponders can receive a sufficient power from this radiated magnetic field, which can pose problems of conflict in the data transmissions and/or can result in an unauthorized pirating of the data transmissions between a transponder and a read/write terminal.